Question

Examination of the histone modifications of a gene reveals an abundance of histone H3 with lysine 27 modified with a single methyl group. Does this suggest that this gene is activated or repressed? How would your answer change if many lysine 27 residues were modified with three methyl groups each?

Answer 1

This gene is likely activated since the mono-methylation of the Lysine residue at position 27 in histone H3 is a mark of active transcription. A trimethylation of the same residue would indicate a silenced/repressed gene.

The trimethylation of the Lysine residue at position 27 on Histone H3 is a repressive mark, deposited by the activity of the Polycomb Repressive Complex. This epigenetic mark is often represented in literature as H3K27me3.

The mono-methylation of the same residue is an Activator mark. This epigenetic modification is shortened to H3K27me.